1. Field
This disclosure relates generally to input buffer circuits, and more specifically input buffer circuits for receiving digital data transmissions.
2. Related Art
In high speed data communication systems, data interconnect systems behave like transmission lines when transmitting data, in the form of electric signals, from one point to another. As the electric signals are transmitted, they can be modified by the electrical properties (e.g., the transfer characteristics) of the transmission line or transmission line system. When electric signals are received, an input buffer circuit (i.e., a buffer circuit operating as an input to another circuit) can be used to interpret the digital data by determining whether a high level signal or a low level signal has been received, which signal levels can represent digital data interpreted as ones or zeros, or high logic values or low logic values.
The operation of the buffer circuit can improve the rejection of noise in the digital signal. For example, hysteresis can be used to clean up the waveform. However, conventional hysteresis input buffers can introduce undesirably long propagation delays when receiving a stair-stepped waveform. The stair-stepped waveform can be produced in an asymmetric transmission line system, or multiple-point transmission line network, due to forwarding and reflections. The forwarding and reflections can be caused by a transmitted signal being distributed to two different receivers that are different distances from the signal transmitter. In one case, a reflected signal generated at a more distant receiver can influence a rising or falling signal at the input of a closer receiver (i.e., less distant from the signal transmitter), thereby causing a stair stepped waveform at the input of the closer receiver. The stair step in the waveform introduces an undesirable delay as the signal transitions from low to high, or high to low, logic values. Note that the delay caused by the stair-stepped waveform can also be a problem for the widely used input buffers that do not use hysteresis, wherein a switch point threshold is set near 50% of the input level range.